Self-sustaining highly porous regenerable thermoplastic fiber mat



SELF-SUSTAINING HIGHLY POROUS REGENEHABLE THERMOPLASTIC FIBER MAT FiledMarch 6, 1963 April 1968 c. L. NOTTEBOHM EII'AL 3,378,398

I." z I". a) Q CARL LUDWIG NOTTEBOHM ROBERT SCHABERT ALBRECHT BURK BYBunezss Dmfuez ATTORNEYS.

United States Patent Office 3,378,398 Patented Apr. 16, 1968 6 Claims.oi. 117-140 This application is a continuation-in-part of applicationSer. No. 172,712, filed Feb. 12, 1962 and now abandoned, whichrepresents a division of United States application Ser. No. 836,301,filed Aug. 26, 1959, issued patent No. 3,035,943, which, in turn, is acontinuation-in-part of application Ser. No. 685,859, filed Sept. 24,1957 and now abandoned.

This invention relates to a regenerable fiber mat suitable for thefiltration of gases, scouring operations, paint and wax strippingoperations and abrasive operations.

Non-regenerable fiber mats suitable for the filtration of gases,scouring ope ratio'fis, paint and wax stripping operations, and abrasiveoperations, are described and claimed in several patents. These fibermats, however, have certain disadvantages. First of all, they offer toomuch resistance to the passage of air which is particularlyobjectionable to their use as filters. There exists no uniformly bondedfiber mats having an air permeability which is less than mm. watercolumn at a speed of the streaming air of 1.5 meters per second. Thisvalue which should be substantially maintained even after prolonged useis the lower limit which is required for most technical purposes, e.g.,for the application in the air tunnels in automobiles.

Another disadvantage of these known fiber mats is their relatively highspecific weight and their low resistance against compressing forces sothat they do not impart a suflicient abrasive, scouring or strippingaction. In most fibrous mats, the individual fibers are not or only to avery small extent bonded together in the interior of the fiber mat sothat they are difiicult to clean after use. They need facing sheets onthe opposite surfaces or relatively large amounts of an adhesive must besprayed on one or both surfaces of such non-woven fiber mats in order toeflect a strong bonding of those fibers to one upon another which arepresent in the surface areas. Although said fiber mats containing looseaggregated fibers in the interior of the non-Woven structures can bemade self-sustaining by the application of facing sheets or relativelylarge amounts of an adhesive to one or both surfaces; it isself-explanatory that just for that very rea son, the air permeabilitydecreases, since a large area of the surface will be blocked byimpervious materials.

It is further obvious that the air permeability sharply decreases afterdust particles have deposited on those spots which were still free forthe passage of air. Thus, the mat will be clogged long before anappreciable amount of dust particles has penetrated into the interior ofthe fiber mat. For this reason, it is practically senseless to makethick non-woven fiber mats in an attempt to improve the filtering etfector the dust absorption capacity. Further it is practically senseless tomake thick non-woven fiber mats in an attempt to improve the scouring,abrasive, stripping, etc, effect since the extreme resulting depth hasthe disadvantage that debritus from the floor, articles, etc., beingcleaned (scoured, stripped) passes throughout the interior of the matand is discharged outwardy from the mat edges necessitating additionalclean-up operations.

It is interesting to note that all the known methods for the productionof fiber mats from non-woven fabrics of open filamentary structure whichis necessary for the free passage of air, carefully avoid impregnationof the starting materials, i.e., the unwoven unfelted skeletonframework, under pressure. On the contrary, prior art methods in mostcases spray an adhesive on the surface of said skeleton framework offibers. As previously stated, the droplets of the adhesive depositpreferably on the fibers of the outer parts of the fiber fleece. Only avery small percentage of the adhesive, if at all, will penetrate to theinner parts. The thus resulting porous structure is therefore ofdifferent stability in its various parts. But once the surface cover isdamaged, the whole fiber mat has to be discarded since the fibers of theinner part of the fiber structure are released. Needless to say suchfiber mats cannot be regenerated by washing with water.

The last-mentioned disadvantage may of course be overcome byimpregnating the porous skeleton framework of fibers in random directionwith a liquid adhesive under pressure, for example, by passing theimpregnated fiber web through a pair of rollers, as described in thePatent No. 2,774,687 to Nottebohm, et al. Under these conditions,however, the open porous skeleton framework structure of the drystarting web is entirely destroyed. Only sheet material similar totextile or leather of a maximum thickness of 2.5 mm. may be obtained.Said products are distinguished by valuable properties, such assoftness, porosity, elasticity, springiness, crease-resistance, waterresistance, and fastness against washing and cleaning. Although they mayfilter the air from dust particles, they cannot be employed as gasfilters under the conditions of practice, since their permeability istoo low. Thus, in order to pass air through that leather-like sheet at avelocity of 1.5 meters per second, a minimum pressure diflerence of mm.water column is necessary. Also their dust, grit, dirt, etc., storingcapacity is poor.

Any of the prior art references in this field recommends the applicationof aqueous rubber latex or aqueous adhesives to which a wetting agentsuch as sodium benzenesulphonate has been added. Although said methodsallow a complete through impregnation and in consequence thereof, auniform bonding throughout the entire fleece, the open porous skeletonstructure of the dry fiber fleece cannot be maintained during theimpregnation step. After a complete impregnation in the presence ofwetting agents and under pressure, the thickness of the impregnatedfiber fleece is only about 1 to 5 percent or even less of the original,dry fiber fleece. Those skilled in the art call this phenomenacollapsing of the fleece.

The only way to prevent collapsing of the skeleton structure of the dryfiber fleece consisted in spraying the adhesive on the dry fleecewithout subjecting it to pressure. The disadvantages of said method havebeen previously stated.

It is now one object of this invention to make highly porous,self-supporting stable, substantially non-compressible fiber mats whichmay be regenerated by washing with water.

Still another object of the invention is to provide a process wherebythe skeleton structure of a dry fiber fleece is substantially maintainedduring the step of a thorough impregnation with a liquid adhesive underpressure.

A still further object of the invention is the production of regenerablefiber mats suitable for the filtration of gases in which any fiber isbonded to the neighboring fiber with the same strength, therebysubstantially maintaining the skeleton structure of the dry fiber fleeceand the high air permeability even after prolonged use.

A still further object of the invention is the production of fiber matssuitable for the filtration of gases, the thickness of which afterimpregnation under pressure is at least 30 percent of the thickness ofthe starting dry fiber 3 fleece, and the specific weight of which isabout 0.007 to 0.05.

A still further object of the invention is the production of fiber matssuitable for scouring, stripping, and abrasive operations as encounteredin both domestic and commercial use, the thickness of which afterimpregnation under pressure is at least 30 percent of the thickness ofthe starting dry fiber fleece, and the specific weight of which is about0.007 to 0.05.

The above and related objects will appear more clearly from thefollowing description of typical ways and means of obtaining theobjectives referred to, which description has reference to the appendeddrawings wherein:

FIGS. 1 and 2 are explanative and show preferred embodiments of theimpregnation step.

FIG. 3 is an enlarged detail view illustrating the three dimensionrandom arrangement of the fibers after having passed the squeezingrollers 3, 3 and a heating zone (not shown).

In accordance with the present invention, we have found that if aspecial combination of solvents and adhesives is used for theimpregnating of a dry fiber fleece, the skeleton structure of saidfleece may be substantially maintained even during a throughimpregnation under pressure.

The left column of the following table shows suitable adhesives that maybe dissolved in any of the solvents or mixtures thereof listed in theright column.

Adhesive: Solvent Polyamide Butylalcohol. HexamethylolmelamineMethylalcohol. Triphenylmethanetriisocyanate 4,4',4 Benzene.Diisocyanates, e.g., O=@N-C H CH Methylenechloride.

C H -N C=O Trichloroethylene.

Acetone.

more than a definite amount of water cause the collapse of the skeletonstructure of the dry fiber fleece. Although there is a very sharp limitfor each solvent or solvent mixture, it is not possible to state ingeneral terms that amount of water which is maximal permissible in saidsolvents or mixtures thereof, since said value is different in eachspecific case.

It can be said, however, that none of the above-mentioned solvents orsolvent mixtures may contain more than 50 percent of water. Furthermore,only such watercontaining solvent combination can be used that form ahomogeneous liquid and in which no wetting agents are present.

The starting material for the production of the novel fiber matsconsists of a batt of cardable fibers. The fibers include fibers ofvegetable origin, such as cotton ramie, flaked bast, fibers of animalorigin, such as sheep wool, synthetic fibers, such as viscose rayon,copper rayon, acetate rayon, polyamides, polyvinylchlorides,polyvinylidene chlorides, polyacrylonitriles, polyvinyl alcohols,polyethylenes, polyesters, among these also synthetic protein fibers,e.g., Merinova, Ardil, Vikare or fibers produced from Merinova is asynthetic casein fiber (see Textile Fiber from Casein, R. F. Peterson,et al., Industrial & Engineering Chemistry, vol. 37, 19-25, p. 492).Ardll is a wool-like fiber made from peanut protein. Ardil is theregistered trade name of Imperial Chemical Industries. Vikare is a p infi copolymerizates; included furthermore are mixtures of these fibersamong themselves and with other fibers, Cardable fibers, i.e., thosehaving a length of at least 4 mm. are first formed into a loose fleecein which the fibers are randomly arranged in intersecting directions,i.e., polyposed. This loose fleece should have a thickness of at leastgreater than 12 mm., and preferably at least 25 mm., and may be producedin any conventional or known manner. This fleece may be produced, forexample, by forming thin, carded layers or Webs of the fabric on aconventional carding machine, and laying the webs one across the other,so that the direction of the fibers crisscross eac other, inintersecting directions. The fleece may also be produced in any otherknown manner, as for example, by means of the random web process.

Prior to the impregnation and compression as shown in FIGURES 1 and 2,it has been found preferable to prestabiiize one or both surfaces of thefleece. This may be done, for example, by applying a small quantity ofthe bonding agents to the surface or surfaces to be stabilized, as forexample, by spraying or brushing, and by setting this bonding agent inthe conventional manner to provide for the prestabilization. Theapplication of the bonding agent for the prestabilization should beeffected in such a manner that the inter-spaces between the fibers ofthe surface layers are preserved and are not filled by the bondingagent. The amount of the bonding agent necessary for theprestabilization is about 10 percent of the total amount. Afterapplication of the prestabilization layer (e.g., by foaming, spraying)follows a drying step which may be performed according to conventionalmcthods, e.g., by passing the thus treated fleece through a dryingdevice. Other methods for the prestabilization are described in Example3.

Referring to FIG. 1 of the drawing, the thick fleece 1, produced in themanner described above, which is prestabilized on its surface 2 in themanner described above, is passed through the nip of the roller pair 3,3. The space above the nip between the rollers is filled with theimpregnating agent 4 which uniformly impregnates the fleece 1 before andduring its passage through the roller nip. The impregnated fleece 5leaves the roller nip in a strongly compressed state, but expands andbulges out at 6 to a volume which is at least one-third of the thicknessof the starting fleece. The expanded fleece structure is then conductedthrough a drying chamber (not shown), in which the volume and structureare fixed and stabilized by a setting of the bonding agent in theimpregnation liquid.

In the embodiment shown in FIG. 2, the fleece 1, prepared as describedabove, has both surfaces 2, 2' prestabilized in the manner describedabove, by treatment with small quantities of the bonding agents Whilepreserving the interstices between the fibers. The fleece is thenconducted through an impregnating bath 7, in which it is uniformlywetted by the impregnating agent. From the impregnating bath, the fleeceis guided vertically upward through the nip of the rollers 3, 3'. Uponpassing through the roller nip, the impregnated fleece is freed fromexcess impregnating agents by the squeezing action and compressed. Thestrongly compressed fleece which emerges at 6, expands and bulges outagain on its way to the drying chamber where it is fixed and stabilizedby drying.

FIGURE 3 is an enlarged detail view of the fiber mat made according tothe above-described method. The adhesive 8 deposits in the form ofdroplets in hap-hazard manner, preferably, however, at the intersectionof the two fibers, thus cementing them together. It should be emphasizedthat said detail view may be obtained by enlarging any section of thefiber mat, since there is a uniform bonding throughout the entire mat.The amount of adhesive incorporated into the fiber mat neither improvesnor deteriorates the filtering, scouring, stripping, and/ or abrasivecapacity of the final fiber mat. The adhesive is only necessary tostabilize the open skeleton structure of the originally dry fiber fleecein which the individual fibers are loosely aggregated. It is not thepurpose of the invention to incorporate a certain amount of an adhesiveinto a fiber fleece but to incorporate an adhesive under such conditionsthat the open porous skeleton structure of the dry fiber fleece and itsoutstanding filtering, scouring, stripping, etc., characteristics aresubstantially maintained during the impregnation with an adhesive.Although it is impossible to fully maintain the thickness of the dryfiber fleece during the above-described impregnation under pressure, ourmethod provides a considerable improvement in this respect since thethickness after impregnation still amounts to at least 30% of theoriginal thickness of the starting fiber fleece, whereas the thicknessof any fiber fleece impregnated by any other method is about 1 to 5% ofthe dry fiber fleece, thus furnishing structures the porosity of whichis too low for filter purposes.

EXAMPLE 1 A fiber fleece or web, prepared by means of carding,consisting of 300 grams per square meter polyamide fibers of 60 mm.staple length and denier having a web thickness of 50 mm. and a specificgravity of 0.006, was coated uniformly on one side with the use of adoctor blade with a foam mixture consisting of a copolymer of polyvinylchloride acetate of the following composition:

Composition Solid Liquid Aqueous emulsion containing 50% of acopolymerizate of equal parts of polyvinyl-chloride and polyvinylacetate, parts by weight 50 100 Water, parts by weight 795Sodiumbenzene-sulphonate, aqueous solution, parts by weight 3. 7 5

Polyamide (solid) 28 Solvents:

Methyl alcohol 70 Benzene Water 10 and passed intoa squeezing deviceconsisting of two rolls spaced 0.5 mm. apart. The access solution wassqueezed off until a wet pickup of 100%. After leaving the roll nip, theimpregnated fiber web expanded to of the original volume and wassubjected for 12 minutes to a drying process at 115 C. The content ofbinder, calculated on the weight of the dry original web mat was 28%,the final fiber mat had a specific gravity of 0.036, a thickness of 12mm. (which therefore had dropped to 33% of the value it had before thethorough impregnation). The compressibility under a load of 10 kg. persquare meter is not measurable. The air resistance at an air velocity of1.5 meters per second is 2.8 mm. water column.

EXAMPLE 2 Hexamethylolmelamine 55 Diammoniumphosphate 5 Water 23 Methylalcohol 17 Squeezing out is effected by a pair of rolls having a nip of0.4 mm. until a wet pickup of 120%. The fiber web from which the excessbinder has been removed is dried and condensed for 12 minutes at atemperature of 130 C.

The final product has the following properties:

Thickness 17 mm. (47% of the prestabilized web). Specific gravity 0.028.Compressibility under a load of 10 kg. per sq. meter Not measurable. Airresistance at an air velocity of 1.5 meters per second 4.1 mm. watercolumn.

EXAMPLE 3 A fiber web or fleece, consisting of 92 parts of polyesterfibers, 60 mm. staple length, 22 denier and 8 parts of polyvinylchloride fibers, 40 mm. staple length, 3 denier, weight of fiber web 250grams per square meter, is subjected for 6 minutes to a temperature of115 C. and in this manner prestabilized sufficiently throughout byshrinkage of the polyvinyl chloride fibers so that impregnation by asqueezing device is made possible. The fiber Web prestabilized in thismanner is impregnated with a binder solution of the followingcomposition:

Parts Triphenylmethanetriisocyanate 4,4',4 in a solution ofmethylenechloride, concentration 20% 316 Polyester of 3 mol adipic acid,3 mol butyleneglycol and 1 mol glycol, sold as Desmophene 800, in asolution of trichloroethylene concentration 50% 200 The fiber Web isfreed from the excess by a pair of rolls (roll nip 0.4 mm.). The wetpick-up is The impregnated fiber web expands to a web thickness of 28mm. and is now subjected to drying at 15 C. for 12 minutes. The producthas the following properties:

Thickness 17 mm. Specific gravity 0.019. compressibility under a load of10 kg. per sq. meter 4%. Air resistance at an air velocity of 1.5 metersper second 2.1 mm. water column.

EXAMPLE 4 A fiber fleece, consisting of 380 grams per square metersheeps wool, Type B, web thickness 80 mm., is pre-stabilized on one sideas in Example 1. The web pre-stabilized in this manner is impregnatedwith a solution as in Example 1, and fed to a squeezing deviceconsisting of two rubber rolls; the wet pickup is 80%. The impregnatedfiber web expands after leaving the squeezing device to a thickness of33 mm. and is dried at C. for 12 minutes. The product has the followingproperties:

Thickness 33 mm. Specific gravity 0.014. Compressibility under a load of10 kg. per sq. meter 10%. Air resistance at an air velocity of 1.5meters per second 4.8 mm. water column.

EXAMPLE 5 A fiber web or fleece, consisting of 250 grams per squaremeter spun rayon viscose, 60 mm. staple length 22 denier and 60 mm.thickness, is uniformly mixed during the carding with 50 grams ofpolyethylene powder and thereupon subjected to a temperature of C. Thepolyethylene powder introduced, due to its plasticity at the existingtemperature, effects a slight bonding together of the fibers. The fiberweb which is in this way slightly stabilized through and through has athickness of 55 mm. and is now subjected to the impregnation process,with the solution used in Example 1. The excess binder is squeezed outby a pair of rolls. The nip is 0.4 mm., and the wet pickup 80%. Theimpregnated web is thereupon dried for 12 minutes at 115 C. The producthas the following properties:

Thickness 50 mm. Specific gravity 0.007. Compressibility under a load of10 kg. per sq. meter 6%. Air resistance at an air velocity of 1.5 metersper second 2.1 mm. water column.

EXAMPLE 6 A fiber web or fleece, consisting of 150 grams per squaremeter polyarnide fibers, 15 denier, 60 mm. staple length is stabilizedon one side as in Example 1, by the application of a covering foam. Uponintroduction into the impregnating solution a similar web is fed to itin such a manner that the two pre-stabilized sides lie on the outside.The weight of this combined web is then 309 grains, the thickness 36 mm.The impregnating and squeezing are affected as in Example 1. The producthas the following properties:

Thickness 12 mm. Specific gravity 0.033. compressibility under a load of10 kg. per square meter 3%.

Air resistance at an air velocity of 1.5 meters per second 3.0 mm. watercolumn.

The product differs from that of Example 1 solely by the fact that ithas a surface pre-stabiiized on both sides.

We claim:

1.. Self-sustaining highly porous regenerable thermoplastic fiber mathaving a specific gravity between 0.007 and 0.05 and a thickness of atleast 4 mm. and an air permeability which. is less than 5 mm. watercolumn at a speed of streaming air of 1.5 meters per second even afterprolonged use and in which the fibers are bonded together at theircrossing points in any part of said mat with the same strength so thatsolid particles incorporated in the fiber mat may be removed by washingwith water.

2. Self-sustaining highly porous regenerable thermoplastic fiber mataccording to claim 1, wherein said bonding together of saidthermoplastic fibers at their crossing points is effected by an adhesiveselected from the group consisting of polyamide, hexamethylolmelarnine,triphenylmethane-triisocyanate 4,4',4", diisocyanate, andurea-formaldehyde condensation products.

3. Self-sustaining highly porous regenerable thermoplastic fiber mataccording to claim 2, wherein the content of said binder amounts to 28%calculated on the weight of the dry mat.

4. Selfisustaining highly porous regenerable thermoplastic fiber mataccording to claim 1, wherein said thermoplastic fibers are bondedtogether at their corresponding points, the interstices between adjacentfibers being open and substantially unfilled by binder.

5. Self-sustaining highly porous regenerable thermoplastic fiber rnatwhich can be employed in filtering operations having a specific gravitybetween 0.007 and 0.05 and a thickness of at least 4 mm. and an airpermeability which is less than 5 mm. water column at a speed ofstreaming air of 1.5 meters per second even after prolonged use and inwhich the fibers are bonded together at their crossing points in anypart of said mat with the same strength, so that dust particlesincorporated in the fiber mat may be removed by washing with water.

6. Self-sustaining highly porous regenerable thermoplastic fiber rnatwhich can be employed in scouring, stripping, and abrasive operationswithout leaving undesirable residue and which is capable of beingreadily cleaned having a specific gravity between 0.007 and 0.05 and athickness of at least 4 mm. and an air permeability which is less than 5mm. water column at a speed of streaming air of 1.5 meters per secondeven after prolonged use and in which the fibers are bonded together attheir crossing points in any part of said mat with the same strength, sothat solid particles incorporated in the fiber mat may be removed bywashing with water.

References Cited UNITED STATES PATENTS 2,879,197 3/1959 Maskat et al.15679 2,972,554 2/1961 Maskat et al 156-79 2,353,937 7/1944 Smith 5242,734,841 2/1956 Merriman 16l-157 X 2,719,598 10/1955 Lindner 55-524 X2,339,562 1/1944 Eustis 161-170 X 2,958,593 11/1960 Hoover et a1 51295WILLIAM D. MARTIN, Primary Examiner.

MURRAY KATZ, Examiner.

R. HUSACK, Assistant Examiner.

1. SELF-SUSTAINING HIGHLY POROUS REGENERABLE THERMOPLASTIC FIBER MATHAVING A SPECIFIC GRAVITY BETWEEN 0.007 AND 0.05 AND A THICKNESS OF ATLEAST 4 MM. AND AN AIR PERMEABILITY WHICH IS LESS THAN 5 MM. WATERCOLUMN AT A SPEED OF STREAMING AIR OF 1.5 METERS PER SECOND EVEN AFTERPROLONGED USE AND IN WHICH THE FIBERS ARE BONDED TOGETHER AT THEIRCROSSING POINTS IN ANY PART OF SAID MAT WITH THE SAME STRENGTH SO THATSOLID PARTICLES INCORPORATED IN THE FIBER MAT MAY BE REMOVED BY WASHINGWITH WATER.